Within the last few decades there has been a substantial shift in demand away from traditional explosive compositions such as TNT, dynamite, and nitroglycerin for hard rock mining, excavation, and similar commercial blasting, in favor of cheaper, readily available inorganic salts such as prilled ammonium nitrate (AN) and also mixtures of such salt with other inorganic oxygen-supplying salts and an organic fuel or oil (ANFO).
Such formulations are relatively inexpensive and, when desired, can be manufactured "in situ" in relative safety compared with most traditional explosives, to avoid a number of problems generally associated with long term explosive storage and transportation.
When the bore hole is very deep and/or wet, however, AN and ANFO formulations are less attractive since they are easily desensitized and must be protected (a) by special packaging with attendant cost and oxygen-balance problems, or (b) by the addition of expensive coatings, thickeners or gelling agents, and the like. Moreover, ANFO has a somewhat limited specific energy due to low bulk density.
Some of the above-listed problems are partially avoidable by using special slurries having higher bulk densities, but such formulations are still not water proof, and must rely heavily upon gassification or other known density control means to retain even a minimally acceptable level of sensitivity. Pressure conditions within deep bore holes, for example, cause compression of gas voids, often resulting in retention of unexploded charges in difficult-to-reach areas where further drilling, mining or excavation may be necessary.
A substantial breakthrough with respect to moisture resistance is described, for instance, in U.S. Pat. No. 3,161,551 of Egly; here a water-resistant blasting agent is obtained having at least one solid prilled inorganic salt such as "AN" and a 50-70% AN aqueous solution as the aqueous discontinuous phase of an invert emulsion; such agent utilizes carbonaceous fuel oil and an emulsifier, such as a long chain fatty acid or ester derivatives thereof, to form the continuous external hydrophobic phase of the emulsion.
Egly's class of compositions exhibits improved resistance to water because the invert water-in-oil emulsion has the capacity to fill natural voids in the solid prilled inorganic salt component and water cannot easily force its way through the continuous external hydrophobic emulsion phase. Balanced against such advantage, however, are serious sensitivity and storage problems which may be due, in part, to a tendency of the solid oxidizer salt component to promote or encourage desensitizing crystallization within the discontinuous aqueous phase during storage.
Additional variations in the use of oxidizer salts are noted, for instance, in Wade (U.S. Pat. Nos. 4,149,916; 4,110,134) Sudweeks (U.S. Pat. No. 4,322,258) and Jessop (U.S. Pat. No. 4,356,044). Generally speaking, however, such compositions also tend to be deficient in sensitivity characteristics.
Bluhm (U.S. Pat. No. 3,447,978) seeks to avoid Egly's lack of sensitivity through the use of, at least, 4% by volume of occluded or entrained gas within an invert emulsion system consisting essentially of
(1) known water-in-oil emulsifier;
(2) a discontinuous ammonium nitrate aqueous phase (optionally supplemented by other water soluble oxidizer salts) within a continuous organic phase consisting of a carbonaceous fuel having the required predetermined gasretaining consistency at 70.degree. F.; and
(3) the functionally important occluded gas itself inclusive of air or glass bubbles and the like, as a density control agent.
Bluhm's explosive compositions, while capable of avoiding some of the above-listed deep wet-bore hole problems, also suffer from certain stability, sensitivity, and potential pollution problems because the proportion, by weight, of discontinuous aqueous-phase to continuous organic phase must be high in order to even approximate a satisfactory Oxygen Balance.
In the Tomic Patent (U.S. Pat. No. 3,770,522), gas bubbles and other density control agents such as microballoons are required along with a stearate salt as a special emulsifier, plus aluminum, magnesium, and smokeless powder as supplementary fuels in an emulsion-type explosive composition. Such compositions, however, do not produce the desired high volume-energy range.
Efforts to eliminate density control agents such as entrained gases and microballoons in order to increase specific energy output and to reduce cost, have not been commerically satisfactory thus far, because of the general lack of sensitivity of dense AN emulsions, particularly those exceeding about 1.3 gm/ml; moreover, the use of temperature-sensitive explosive sensitizers such as nitroglycerin or formulations containing nitroglycerin generally increase sensitivity at the expense of very desirable safe handling characteristics normally expected of emulsion blasting agents. Sensitizer-activated compositions known to the art are described, for instance, in Berthmann et al (U.S. Pat. No. 3,356,547).
It is an object of the present invention to optimize volume energy and sensitivity of high density invert emulsion blasting compositions substantially lacking in void-producing density control agents while retaining the excellent formulation, handling, and safety characteristics generally attributable to water-in-oil emulsion blasting compositions by adding an effective amount of a modified sensitizing formulation.
If is a further object to optimize volume-energy release characteristics and resistance to deep hole pressure effects while retaining desired sensitivity.